Performance monitoring is an integral aspect of optical networking, being used for both quality assurance and fault localization. While quality assurance monitoring can be confined to the end-terminal, effective fault localization requires that monitoring be done at many locations throughout the network. In the past, this monitoring was done only at OEO (optical-electrical-optical) regeneration sites, where the optical signals were converted back to electrical signals, cleaned up, and then retransmitted optically. The OEO conversion process enabled full access to the data signal in the electrical regime, making performance monitoring straightforward. As optical networks evolve, however, a greater degree of optical transparency is being realized. Examples of this are the emergence of ultra-long haul optical transport and large-scale optical switching. As the degree of optical transparency within networks increases, the distance between OEO sites in the network is increasing, and new methods of performance monitoring (not confined to OEO sites) will be needed to insure effective fault localization.
One technique for optical performance monitoring is optical spectral analysis. This technique is commonly used in wavelength division multiplexed (WDM) transmission systems. A small portion (˜1–5%) of the total optical power being transmitted in the fiber is tapped and sent to an optical spectrum analyzer. The analyzer measures the optical power as a function of wavelength, and thus provides information on the presence or absence of specific WDM channels and their respective power levels. In addition, the regions of the spectrum lying between the channels can be used to derive an estimate of the optical noise present in the system, providing a per-channel measurement of the optical signal-to-noise ratio (OSNR) present in the system.
The OSNR measurement provided by spectral analysis has several important limitations. For high spectral efficiency WDM systems, the optical power measured at the inter-channel wavelengths will contain power from the spectral tails of the adjacent channels, and will not enable measurement of the inherent optical noise floor. In addition, if there are optical add-drop filters present throughout the system, the optical noise measured at the inter-channel wavelengths will not necessarily reflect the in-band optical noise floor. Finally, this measurement is not sensitive to purely in-band noise sources, or to pulse-distortion effects arising from dispersion and fiber nonlinearities that may also adversely affect signal quality.
An emerging technology aimed at addressing these limitations is the reference-receiver Q-monitor (eye-diagram analysis). One such technique is described by W. G. Yang, “Sensitivity issues of optical performance monitoring”, IEEE Phot. Tech. Lett. 14, 107–109 (2002). In this approach, a small portion (again 1–5%) of the optical signal is tapped and sent through a tunable filter to a conventional full-bit-rate receiver. The tunable filter is capable of isolating a single WDM channel, and can be tuned to receive any of the channels being transmitted through the system. The receiver performs full clock and data recovery and can provide detailed information about the quality of the channel. This is done either using format/protocol specific techniques (such as examining SONET parity bits, or looking at the output of forward error correction [FEC] chips) or using a format independent eye-diagram analysis technique.
The main limitations of the reference-receiver approach involve both sensitivity and cost. Because only a small portion of the total optical power is available for detection, and because a conventional wideband receiver is required, the inherent receiver noise places severe limits on the sensitivity of the monitor. This problem can be alleviated by placing an optical pre-amplifier in front of the monitor. However, this adds significant cost. In addition, high-speed optical transmission systems are often designed such that the signal pulses are intentionally broadened by chromatic dispersion throughout network (to reduce the penalties of optical nonlinearities). In that case, the reference-receiver may require dispersion compensation of the signal prior to detection. This also adds significant cost. For any performance monitoring solution, an increase in cost will result in fewer monitors being placed throughout the network, which will reduce the effectiveness of the fault localization.